It is well known in the art to utilize gravity downflow filtration tanks divided into a plurality of filter cells, one adjacent the other, and all containing a multi-layer or single layer arrangement of granular filter media such as sand, gravel and the like. In typical downflow filtration systems of this type, water or other liquid containing suspended solid particles is introduced into the filter basin from above, and clarified water is drawn off from a chamber either beneath or adjacent the individual filter cells. During downward flow through the individual cells, particulate matter is entrapped within the layer or layers of granular filter media, but eventually, the particulate matter clogs the filter media, thereby reducing the filtering capability of the system. Thus, there is a need for a periodic cleaning of the filter cells. This is usually accomplished by backwashing the filter via introduction of backwash water or other liquid into the filter in a direction opposite the normal filtration direction. It is also known to maintain such units in continuous operation during backwash by the use of traveling bridge devices which move from one filter cell to the next, cleaning individual cells while permitting the filtration process to continue in the remaining cells.
Examples of traveling bridge apparatus may be found in U.S. Pat. Nos. 5,041,405; 5,089,117; 4,988,439; 4,957,631; 4,859,330; 4,764,288; 4,617,131; 4,540,487; 4,486,307; 4,133,766; 3,984,326; 2,235,227; and 2,302,449. Typically, traveling bridge systems include an overhead carriage, movable along tracks, guideways or the like, which supports a backwash hood for movement along and over the open upper ends of the filter cells, usually in a step-wise manner. As already mentioned, for a downflow type filter, water or other treatment liquid is caused to enter into the cell being backwashed (by a backwash pump) from below in a counterflow arrangement to the normal filtering direction. The backwash hood typically also includes a suction head (i.e., an additional pump) for drawing out fluid and debris forced to the surface of the filter cell as a result of the backwash. As backwashing of individual cells is completed, the traveling bridge moves the backwash hood over the next adjacent cell.
It is also known to utilize combined air/water backwash techniques in traveling bridge systems. Combined air/water backwash systems are generally used in conjunction with single media, unstratified filter beds, and are usually employed to increase the effectiveness of water only backwash. These systems utilize a backwash pump in combination with an auxiliary air supply such as a blower or compressor. Examples of a combined air/water backwash in a traveling bridge filter may be found in U.S. Pat. Nos. 5,078,873; 5,032,294; 4,624,783 as well as in commonly owned U.S. Pat. Nos. 4,859,330 and 5,401,405.
In U.S. Pat. No. 5,401,405, an improved air/water backwash system is disclosed in which air is drawn into air supply risers in each cell by negative pressure in the cell. This system also incorporates a self-aspirating venturi which expels entrained air or gas through the backwash water discharge pipe.
In commonly owned, U.S. Pat. No. 5,792,359, a celless traveling bridge filter is disclosed, utilizing a plurality of transversely arranged underdrain headers for carrying filtered water to the filtrate channel, and also for introducing backwash water into the filter media, in combination with a unique backwash sealing shoe which prevents escape of backwash water from the bed back to the adjacent filtrate channel by blocking the outlet ports of adjacent headers.